Travelling-wave tube amplifier having asymmetric magnetomotive force within the collector for preventing backflow of secondary electrons



May 20, 1969 susumu sU ETAL 3,445,714

TRAVELLING-WAVE TUBE AMPLIFIER HAVING ASYMMETRIC MAGNETOMOTIVE FORCEWITHIN THE COLLECTOR FOR PREVENTING BACKFLOW OF SECONDARY ELECTRONSFiled Oct. 17, 1966 Sheet of 2 J. ICE-.1- 2

y 1969 ,SUSUMU ,YASUDA ETAL 3,445,714

TRAVELLING-WAVE TUBE AMPLIFIER HAVING ASYM RIC MA ETOMOTIVE FORCE WITHINTHE COLLECTOR FOR PREVE NG BA LOW OF SECONDARY ELECTRONS Sheet 2 of2Filed Oct. 17, 1966 United States Patent Int. Cl. nol 25/34 U.S. Cl.SIS-3.5 18 Claims ABSTRACT OF THE DISCLOSURE A travelling-wave tubeamplifier incorporating a periodic magnetic field focussing assemblywhich includes at least one magnetic member positioned closest to thecollector of the amplifier which is capable of generating anasymmetrical magnetic field pattern for substantially preventingsecondary emission electrons from leaving the collector structure.

The instant invention relates to travelling-Wave tube amplifiersincorporating periodic magnetic field focussing means for focussing theelectron beam of the travelling- Wave tube, and more particularly to atravelling-wave tube amplifier of the type described wherein furthermeans are provided to prevent deterioration of operating characteristicsand eventual destruction of the tube amplifier by exerting suppressivecontrol of the secondary electron emission from the collector of thetube amplifier.

In travelling-wave tube amplifiers of the type described, the directcurrent power consumed by the travellingwave tube may be reduced inorder to raise the operating etficiency of the device. To do this, itbecomes necessary in operation to reduce the collector voltage level sothat it is lower than the voltage of the slow-wave structure provided inthe tube amplifier. With the collector voltage being lower than thevoltage of the slow-wave structure, the secondary electron emissionphenomenon causes sec ondary electrons emitted from the collector to beaccelerated towards the slow-wave structure, thereby substantiallyincreasing the slow-wave structure current. This leads to substantialover-heating of the slow-wave structure, and consequently to thedeterioration of the vacuum condiiion within the tube envelope. Thisdeterioration, in turn, causes various problems such as an increase ofthe ion noise in the structure and a significant reduction in usefuloperating life. Furthermore, the above-mentioned secondary electronscause a significant reduction in the attenuation of the attenuator whosefunction is that of preventing oscillation so as to substantially raisethe likelihood of the amplifier tube to oscillate and further increaseelectron beam noise.

In order to prevent the secondary electrons emitted from the collectorto flow in the rearward direction, a conventional practice has been todeflect and trap the electrons by means of shaping the magnetic shieldof the annular pole piece of the periodic magnetic field device positioned closest to the collector into an asymmetric configuration. It hasbeen found, however, that deflection and capture of the secondaryelectron through the use of this method is unsatisfactory since onlyweak leakage flux is available near the collector of the amplifier tube.As a comparative example, a slow-wave structure of the conventional typetravelling-wave tube amplifier, which does not employ any preventivemeasures to eliminate backflowing secondary electrons and which employsa periodic ICC magnetic field structure, has a slow-wave structurecurrent of 0.40 milliampere. The shaping of the annular pole piece ofthe periodic magnetic field device, which pole piece occupies theposition closest to the collector, into an asymmetrical form having asector-shaped cut with a cen tral angle of is found to reduce theslow-wave structure current to a value of 0.37 milliampere, which is analmost insignificant reduction. It has further been found that, if theannular pole piece disposed closest to the collector is provided with asemi-cylindrical member dis posed within the central opening of theannular pole piece in place of a partially cut-away annular pole piece,this causes a reduction in the slow-wave structure current which, whileless than 0.4 milliampere, is found to be greater than the resultingslow-wave structure current of 0.37 milliampere achieved through the useof a partially cut-away annular pole piece.

The instant invention is characterized by providing a travelling-wavetube amplifier wherein use is made of a periodic magnetic fieldfocussing device which is so designed as to easily and effectivelyprevent the deterioration of the amplifier tube characteristics causedby the secondary electron emission from the collector.

In accordance with the instant invention, there is provided atravelling-wave tube amplifier comprising a. magnetic structure disposedadjacent the collector end of the periodic magnetic field device whichyields a magnetomotive force asymmetrical to the central axis of themagnetic field in order to deflect the electrons and hence significantlyreduce backward current flow. The magnetic structure more specificallymay be a permanent magnet having a substantially asymmetrical shape, orconversely, have a symmetrical shape and being provided with anasymmetrical by-pass element, or alternatively, being asymmetricallymagnetized. Many of the above preferred embodiments produce anasymmetrical magnetic field component significantly greater than thatwhich is capable of being produced through the use of conventionalasymmetrical magnetic shields or asymmetric pole pieces, and furtherwhich is quite advantageous in its ability to deflect and trap secondaryelectrons emitted from the collector so as to prevent secondaryelectrons from flowing backwards to the slow-wave structure.

It is, therefore, one object of the instant invention to provide a noveltravelling-wave tube amplifier structure having a periodic magneticfield focussing assembly which incorporates therewith means forsignificantly reducing direct-current power consumed by thetravelling-Wave tube.

Another object of the instant invention is to provide a novel periodicmagnetic field focussing device for use in travelling-wave tubeamplifiers, and the like, which includes means incorporated within thefocussing device and disposed adjacent the amplifier tube collectorstructure for the purpose of significantly reducing the magnitude ofcurrent in the slow-Wave structure.

Still another object of the instant invention is to provide a novelperiodic magnetic field focussing device for use in travelling-wave tubeamplifiers, and the like, which includes means incorporated within thefocussing device and disposed adjacent the amplifier tube collectorstructure for the purpose of significantly reducing the magnitude ofcurrent in the slow-wave structure, said means being a magnetic devicecapable of generating a magnetic field asymmertical to the field centralaxis so as to effectively deflect and trap secondary electrons emittedfrom the collector.

Yet another object of the instant invention is to provide a novelperiodic magnetic field focussing device for use in travelling-wave tubeamplifiers, and the like, which includes means incorporated within thefocussing device and disposed adjacent the amplifier tube collectorstructure for the purpose of significantly reducing the magnitude ofcurrent in the slow-wave structure, said means being a magnetic devicecapable of generating a magnetic field asymmetrical to the field centralaxis so as to effectively deflect and trap secondary electrons emittedfrom the collector, said means being a magnetic member having anasymmetrical configuration.

Still another object of the instant invention is to provide a novelperiodic magnetic field focussing device for use in travelling-wave tubeamplifiers, and the like, which includes means incorporated within thefocussing device and disposed adjacent the amplifier tube collectorstructure for the purpose of significantly reducing the magnitude ofcurrent in the slow-wave structure, said means being a magnetic devicecapable of generating a magnetic field asymmetrical to the field centralaxis so as to effectively deflect and trap secondary electrons emittedfrom the collector, said means being a magnetic member which isasymmetrically magnetized.

A still further object of the instant invention is to provide a novelperiodic magnetic field focussing device for use in travelling-wave tubeamplifiers, and the like, which includes means incorporated within thefocussing device and disposed adjacent the amplifier tube collectorstructure for the purpose of significantly reducing the magnitude ofcurrent in the slow-wave structure, said means being a magnetic devicecapable of generating a magnetic field asymmetrical to the field centralaxis so as to effectively deflect and trap secondary electrons emittedfrom the collector, said means being a magnetic member having a magneticby-pass element shrouding a portion of said magnetic member so as toresult in the production of an asymmetrical magnetic field.

These and other objects of the instant invention will become apparentupon consideration of the accompanying description and drawings inwhich:

FIGURE 1 is an axial sectional view showing the output side of atravelling-wave tube amplifier designed in accordance with theprinciples of the instant invention.

FIGURE 2 is a cross-sectional view taken along the line 2-2 of FIGURE 1.

FIGURES 2a and 2b are front views showing further alternativeembodiments for the magnetic deflection structure of FIGURE 2.

FIGURE 3 is an axial sectional view showing an alternative embodiment ofthe instant invention.

FIGURE 4 is a cross-sectional view taken along the line 4-4 of FIGURE 3.

FIGURE 5 is an axial sectional view of the output side of still anotherembodiment of the instant invention.

FIGURE 6 is a sectional view taken along the line 6-6 of FIGURE 5.

FIGURE 7 is an axial sectional view of the output side of still anotherpreferred embodiment of the instant invention.

FIGURE 8 is a cross-sectional view taken along the line 88' of FIGURE 7.

FIGURES 8a and 8b are front views showing further alternativeembodiments for the magnetic deflection structure of FIGURE 8.

FIGURE 9 shows an axial sectional view of the output side of stillanother preferred embodiment of the instant invention.

FIGURE 10 is an end view of the preferred embodiment of FIGURE 9 lookingin the direction of arrows 1010'.

Making reference to FIGURE 1, there is shown therein a travelling-wavetube amplifier 1 whose presentation in FIGURE 1 is confined to theoutput end thereof for purposes of simplicity. The output end is shownin sectional fashion in the figure, and is comprised of atravelling-wave tube 2 and a periodic magnetic field device 3. Thetravelling-wave tube 2 is comprised of a vacuumized glass envelope 4containing a helical slow-wave structure 5 and an output high-frequencycoupler 8 comprised of a choke 7 and an antenna 6 secured to choke 7adjacent an opening 7a provided therein.

A collector 10 positioned to the right of high-frequency coupler 8 isprovided with a narrow cylindrical-shaped opening 9 projecting in thedirection of the slow-wave structure. The extension 9 is hermeticallysealed to vacuum envelope 4 at 9a. The radiator structure 11 embracesthe outer surface of collector 10 for the purpose of carrying away heatgenerated by the collector.

The periodic magnetic field device 3 is disposed in a coaxial fashionabout the travelling-wave tube 2, and is comprised of a plurality ofaxially magnetized hollow cylindrical permanent magnet members 12 and aplurality of annular pole pieces 13 interspersed between the permanentmagnet members. A waveguide 14 is inserted between the periodic magneticfield device 3 in the manner shown, and has a configuration whichsurrounds the antenna 6 of the output high-frequency coupler 8.

As can best be seen in FIGURE 2, the permanent magnet member 15 employedas the electron deflection magnetic structure, is positioned in closeproximity to the collector end of the periodic magnetic field device 3.The permanent magnet 15 is a substantially cylindrical-shaped permanentmagnet member having a centrally located opening 15a. The permanentmagnet member 15, however, is modified, prior to assembly into thestructure, so as to produce an asymmetrical configuration by cutting offa side portion 16 thereof so as to yield a resultant structure capableof producing an axially asymmetric magnetic field relative to thelongitudinal axis of the structure.

The structure, as shown in FIGURES 1 and 2, has been incorporated into alarge-output travelling-wave tube, and it has been found to result in asignificant reduction in the backward flow of secondary electronsemitted from the collector, as' well as a significant reduction inirregular noise caused by the secondary electron emission.

Insofar as the specific planar configuration of the magnetic member 15is concerned, it should be noted that member 15, which serves as theelectron deflection maga netic structure, may be provided with otherconfigurations which serve to produce as asymmetrical magnetic filedrelative to the longitudinal axis. For example, the magnet may beprovided with a plurality of cut-away portions such as the cut-awayportions 16 and 16a, shown in FIGURE 2a. In addition thereto, more thanone magnetic member having such a cut-away portion may be provided. Forexample, the magnetic member 12 may also be provided with a cut-awayportion 16 in the same manner as the magnetic member 15, shown in FIGURE2. Also, the plurality of magnetic members may be provided withadditional cut-away portions, as shown in FIGURE 2a. Likewise, the shapeand the dimensions of the cut-away portion, or portions, may assume avariety of configurations so long as the desired axially asymmetricmagnetic field is obtained. For example, the magnetic member 15 may beprovided with one cut-away portion 1617, as shown in FIGURE 2b, or maybe provided with a second cutaway portion 16b', as shown by the dottedlines of FIG- URE 2b. As a further means of improving the effectivenessof the deflection structure, it is advantageous in some applications toremove the annular pole piece positioned closest to the collector with aview to further strengthening the axial asymmetry of the magnetic field.For example, in the embodiment of FIGURE 1, it is possible to eliminatethe annular pole piece 17 to enhance the asymmetrical magnetic fieldpattern.

To further emphasize the technical merits of the instant invention inconjunction with the preferred embodiment of FIGURES 1 and 2,experimentation was conducted with a travelling-wave tube amplifierhaving a 6,000 megacycle operating frequency, a collector voltage of2,000 volts, a slow-wave structure voltage of 3,400 volts, a collectorcurrent of 40 milliamps, and a high-frequency out put of 20 watts. In anembodiment having the above operating characteristics and which did notincorporate the structure of the instant invention, the slow-wavestructure current was found to be 0.28 milliamps. By modifying thetravelling-wave tube amplifier to incorporate a cut-away magnet 15having an outer diameter of 34 millimeters, an inner diameter of 15millimeters, a length of 9 millimeters, and a cut-away portion ofsegment shape in cross-section whose chord 16 was positioned distanceD=9 millimeters from the central axis, the slow-wave structure currentwas found to be 0.15 millampere, resulting in almost a 50% reduction incurrent magnitude.

Referring to FIGURES 3 and 4, there is shown therein a second preferredembodiment of the travelling-wave tube amplifier wherein the electrondeflecting magnetic structure, which is disposed at the collector end ofthe perodic magnetic field focussing device 31, is comprised of anaxially magnetized permanent magnet member 19 having a centrally locatedopening 19a and a circular periphery forming a hollow cylindricalmagnetic member. The magnetic by-pass 20 having an arcuateconfiguration, shown by the cross-sectional view of FIGURE 4, isdisposcdaround a portion of the periphery portion of magnet 19. Thearcuate magnetic by-pass 20 acts as a partial shunting device betweenthe poles of the magnet 19 so as to yield a magnetic field having axialasymmetry along the axis of the apparatus. With the partial shuntaligned in the manner shown in FIGURES 3 and 4, it should be noted thatthe electron beam undergoes deflection which occurs in a directionperpendicular to the plane of the FIGURE 4 drawing.

Insofar as the embodiment of FIGURES 3 and 4 is concerned, the followingcomparative data was obtained:

A travelling-wave tube amplifier of the type set forth in conjunctionwith that used in determining the technical merits of the firstpreferred embodiment (of FIGURES 1 and 2) was employed and was found toyield a slowwave structure current of 0.28 milliampere when theoperating values as set forth previously were applied to the assemblyunder test, and when no corrective measures were employed for thepurpose of reducing slow-wave structure current.

Through modification of the travellingwave tube amplifier so as toinclude a hollow cylindrical permanent magnet 19 having an outerdiameter of 34 millimeters, an inner diameter of 15 millimeters, and alength of 9 millimeters; and a magnetic by-pass 20 formed of pure iron,having a wall thickness T of 0.8 millimeter, a width W of 9 millimetersand a length D along the arc of 35 milli meters, was found to yield aslow-wave structure current of 0.13 milliampere which is greater than a50% reduction in current magnitude. It was further found that thetravelling-wave tube amplifier would not oscillate even when the inputor the output high-frequency circuits were short-circuited, and alsothat greatly improved impedancematching characteristics were evidentduring operation of the device.

Whereas the by-pass device 20 may be formed as a separate element of thestrructure, it should be noted that an alternative and satisfactoryapproach is that of forming the magnetic by-pass 20 integrally witheither one of or both of the annular pole pieces 13' and/ or 17, asshown in FIGURE 3.

Referring now to FIGURES 5 and 6, there is shown therein a thirdembodiment of the instant invention which is substantially identical tothat shown in FIGURES 3 and 4, except that the magnetic by-pass 20positioned around the periphery of the permanent magnet member, isreplaced by a magnetic by-pass member 21 which is disposed within theinner periphery of permanent magnet 19. The arcuate-shaped magneticby-pass 21 is thus axially and asymmetrically arranged between the polepieces 13' and 17 in surface contact with the side faces of magnet 19 soas to serve as a partial shunt between both poles of the magnet, andthereby yield a magnetic field along the axis having an axialasymmetrical pattern.

Experimentation with this embodiment was found to yield resultssubstantially similar to those obtained through use of the embodiment ofFIGURES 3 and 4 so as to thereby yield substantially similar technicalmerits thereto.

In much the same way as that described with respect to the embodiment ofFIGURES 3 and 4, the magnetic bypass member 21 may be integrally formedwith one or both of the annular pole pieces 13' and 17.

As a further alternative, it should be noted that the magnetic by-passmember 20 of FIGURE 4 (or 21 of FIGURE 6) need not be a single member,but as a substitute, plural by-pass members may be employed and disposedabout the permanent magnet 19, with the only requirement being that theaxial asymmetrical magnetic field pattern by maintained.

The magnetic material employed in forming the bypass member (20 or 21)may have any suitable magnetic characteristics. For example, the instantinvention is useful even when a material exhibiting a negativetemperature coeflicient of magnetic permeability is employed with a viewto producing temperature compensation of the axial magnetic field. Morespecifically stated, the magnetic flux produced by a permanent magnetgenerally decreases with an increase in temperature so that the axialmagnetic field produced by the magnetic structure employed for electrondeflection along the axis also decreases with this rise in temperatureto cause a fluctuation in the effects of focusing of the electron beamand further of preventing backflow of secondary electrons. In order tocompensate for such fluctuations in magnetic field strength, oneeffective measure which may be employed is set forth in detail incopending US. patent application Ser. No. 576,069, now abandoned, whichdescribes in detail the use of tubular temperature compensation membersformed of a magnetic material having a negative temperature coeflicientof magnetic permeability. Such temperature compensation tubes aredisposed between successive pairs of pole pieces 13 so as to shunt therespective permanent magnets. Thus, while temperature increases,decrease in the magnetic field strength of the permanent magnet membersaccompanying permeability changes occur in the temperature compensationtubes, causing the magnetic field branching through the tubes to bediverted toward the central axis of the amplifier tube apparatus, andhence to maintain a substantially constant magnetic field strengthpattern. Use of a similar magnetic material for the by-pass members 20(or 21) will yield a travelling-wave tube amplifier having stablefocussing action for the electron beam as well as a deflection structurewhich prevents backflow of secondary electrons irrespective oftemperature changes.

FIGURES 7 and 8 show a fourth embodiment of the travelling-wave tubeamplifier structure employing an axially transversely magnetized hollowcylindrical permanent magnet member 22 for the electron deflectionmagnetic structure. The axially transversely magnetized hollowcylindrical permanent magnet 22 is secured to the right-hand face 23 ofannular pole piece 17 which is disposed at the collector end of theperiodic magnetic field focussing device 3. A non-magneticannular-shaped support 24 maintains the magnet 22 a spaced distance frompole piece 17 so that this pole piece will not act as a shunting memberfor magnet 22. The magnetic poles for member 22 are indicated in FIGURE7. The axially transverse magnetic field is generated in the region ofthe tubular-shaped extension 9 of collector 10, with the magnetic fieldlines being substantially perpendicular to the sheet of FIGURE 8.

The permanent magnet member 22 need not be a complete cylindricalmember, and may be substituted by a horseshoe-shaped member 22', shownin FIGURE 8a, or a semicircular shaped member 22", shown in FIGURE 8b,with the poles being arranged as shown in these figures.

FIGURES 9 and 10 show still another preferred embodiment of the instantinvention wherein the travellingwave tube amplifier structure 1 isprovided with a pair of bar magnets and 26 substituted for the axiallytransversely magnetized hollow cylindrical permanent magnet 22 describedin the embodiment shown in FIGURES 7 through 8b. The bar magnets 25 and26 are secured to the right-hand surface of annular-shaped pole piece 17by means of non-magnetic supports 27 and 28 which maintain bar magnets aspaced distance from the surface 23 and hence from the pole piece 17 soas to cause the bar magnets to be substantially unaffected by pole piece17. In addition thereto, the bar magnets 25 and26 are so positioned asto concentrate the magnetic field pattern developed by the magnets inthe immediate region of the extension 9 of collector 10. ConsideringFIGURE 10, it can be seen that the bar magnets are aligned along thediameter d of pole piece 17 such that poles of opposing polarity arearranged in face-to-face relation, producing a magnetic fieldperpendicular to the axis of the amplifier device. The magnetic fieldlines produced in the region of extension 9 are thereby parallel to theplane of FIG- URE 9 so as to yield deflection characteristics which aresubstantially similar to those produced by the corresponding magneticfield of the embodiment of FIGURES 7 through 8b. As a furtheralternative, the pair of bar magnets, shown in FIGURES 9 and 10, may bereplaced by a single bar magnet positioned either in the location of barmagnet 25 or bar magnet 26. In addition thereto, it should be noted thatwhether a single bar magnet is employed or whether a pair of bar magnetsis employed, alignment may be affected along any diameter of the polepiece 17, the only requirement being that this diameter be alignedtransverse (and preferably perpendicular) to the tube longitudinal axis.

All of the embodiments of FIGURES 1 through 10 described hereinsubstantially operate in the following manner:

The electron beam 29 in any one of the embodiments moves from theleft-hand end of the travelling-wave tube amplifier (not shown forpurposes of simplicity) toward the right-hand or collector end thereof.The electron beam is substantially confined to the circular-shapedregion between the helical-shaped slow-wave structure 5. The electronbeam passes through the opening 7a and subsequently through the regionof extension 9 provided on collector 10. In this region, the electronsare deflected away from and caused to deviate from the amplifierlongitudinal axis under the action of the magnetic force provided by thedeflection device of the instant invention which produces a magneticfield substantially perpendicularly aligned to the trajectory of theelectrons, which field is an axially asymmetric magnetic field having afield component aligned orthogonal to the axis. Inasmuch as there is 110focussing magnetic field within collector 10, the electron beam 29diverges due to the space charge force and the electrons are therebycaptured by collector 10 over a wide area of its interior surface 30.The travelling velocity of the electrons cause the electrons impingingupon the interior surface 30 to produce secondary electron emission.Although a portion of the secondary electrons produced by the electronbeam impinging upon surface 30 with high velocity are again captured bythe collector interior surface 30, the remaining electrons which are notso captured and which have velocity components directed toward slow-wavestructure '5, are deflected perpendicularly to the axis when theyapproach the region of influence of the magnetic deflection structuresuch that the component of magnetic field produced by the electrondeflection magnetic structure, which is orthogonal to the axis, causesthe secondary electrons to be deflected and captured by the interiorsurface of extension 9 provided on collector 10. Thus, it can be seenthat the magnetic deflection structure produces a very significantreduction in the amount of secondary electrons flowing back toward theslowwave structure 5 and the choke 7, further resulting in a significantreduction in the deterioration of various characteristics of theamplifier tube caused by the secondary electrons. Furthermore, theinstant invention has the additional advantage of significantly reducingthe harmful local heating of the collector, due to the fact that theelectron beam 29 are captured by the collector over a wide area ofinterior surface 30 so as to greatly reduce the power consumption perunit surface area.

Whereas the magnetic field produced by the electron deflection magneticstructure of the embodiments of FIGURES 7-8 and 9-10 are described ashaving a magnetic field component which is orthogonal with the tubelongitudinal axis, it should be further understood that this magneticfield component need not intersect the tube longitudinal axis at rightangles, but may intersect at any other angle other than a right angle solong as the resulting magnetic field has a magnetic field component of astrength sufficient to deflect the electrons.

Although this invention has been described with respect to its preferredembodiments, it should be understood that many variations andmodifications will now be obvious to those skilled in the art, and it ispreferred, therefore, that the scope of the invention be limited not bythe specific disclosure herein, but only by the appended claims.

The embodiments of the invention in which an exclusive privilege orproperty is claimed are defined as follows:

1. :In a travelling-wave tube amplifier having a travelling-wave tubeincluding a collector at one end thereof, and a periodic magnetic fielddevice disposed coaxially about the travelling wave tube, theimprovement comprising an electron deflection magnetic structurepositioned at the collector end of said periodic magnetic field deviceand adjacent the collector of said travelling-wave tube; said magneticstructure comprising a magnet for producing a magnetomotive force withinsaid collector which force is asymmetric with respect to the axis of themagnetic field produced by said periodic magnetic field device, wherebythe secondary electrons emitted from said collector are prevented fromflowing back toward the slow-wave structure disposed on the side of theopening of said collector.

2. A travelling-wave tube amplifier set forth in claim -1 wherein saidelectron deflection magnetic structure is an axially magnetized hollowcylindrical permanent magnet formed into an axially asymmetric form bycutting away a side portion thereof.

3. A travelling-wave tube amplifier set forth in claim 1 wherein saidelectron deflection magnetic structure is an axially magnetized hollowcylindrical permanent magnet having a circumferentially partial magneticby-pass member disposed along a portion of the periphery of said magnet.

4. A travelling-wave tube amplifier as set forth in claim 1 wherein saidelectron deflection magnetic structure is an axially magnetized hollo'wcylindrical permanent magnet having a circumferentially partial magneticby-pass member disposed along a portion of the internal periphery ofsaid magnet.

5. A travelling-wave tube amplifier as set forth in claim 1 wherein saidelectron deflection magnetic structure is a hollow cylindrical permanentmagnet which is magnetized so as to produce a magnetic field componentorthogonally intersecting the axis.

6. A travelling-wave tube amplifier as set forth in claim 1 wherein saidelectron deflection magnetic structure is a bar magnet producing amagnetic field component orthogonally intersecting the axis.

7. A travelling-wave tube amplifier as set forth in claim 3 wherein themagnetic by-pass is formed of a magnetic material having a predeterminednegative temperature coeificient of magnetic permeability to compensatefor the reduction in magnetic strength of said permanent magnet withincreasing temperature.

8. A travelling-wave tube amplifier as set forth in claim 4 wherein themagnetic by-pass is formed of a magnetic material having a predeterminednegative temperature coefficient of magnetic permeability to compensatefor the reduction in magnetic strength of said permanent magnet withincreasing temperature.

'9. A travelling-Wave tube amplifier comprising: a travelling wave tubehaving a longitudinal axis and being comprised of an elongated helicalslow-wave structure surrounding said longitudinal axis;

a vacuumized envelope having a collector structure at one end thereofand containing said slow-wave structure.

an electron beam source for directing an electron beam along saidlongitudinal axis toward said collector;

an elongated periodic magnetic field focussing device disposed coaxiallyabout said envelope between said electron source and said collector;

means for deflecting said electron beam positioned adjacent the end ofsaid focussing device adjacent said collector;

said deflection means including a magnet for producing a magnetic fieldwithin said collector having a magnetic field component perpendicular tosaid longitudinal axis to prevent secondary electrons emitted from saidcollector from returning to said slow-wave structure.

10. The amplifier of claim 9 wherein said collector is a hollowtubular-shaped metallic member having an opening at a first end forreceiving said electron beam and being sealed at its second end;

a first portion of said tubular collector adjacent said opening beingnarrower than the remaining tubular portion of said collector;

said envelope being joined to the exterior surface of said collector toeffect said vacuumized condition.

11. The amplifier of claim 10 wherein said deflecting means is asubstantially disc-shaped permanent magnetic member having a centrallylocated aperture surrounding said collector first portion.

12. The amplifier of claim 11 wherein said permanent magnet member has aportion thereof removed substantially along a chord of said periphery;said chord intersection the periphery of said aperture at at least twopoints.

155. The amplifier of claim 11 wherein said permanent magnetic memberhas at least one pie-shaped segment removed from said disc.

14. The amplifier of claim 11 further comprising a thin walled,arcuate-shaped by-pass member sheathing a portion of the outer peripheryof said disc-shaped member.

15. The amplifier of claim 11 further comprising a thin walled,arcuate-shaped by-pass member sheathing a portion of the inner peripheryof said disc-shaped member.

16. The amplifier of claim 11 wherein said permanent magnet ismagnetized in a direction transverse to said longitudinal axis toproduce a magnetic field orthogonal to said axis.

17. The amplifier of claim 10 wherein said deflecting means is asubstantially semicircular-shaped permanent magnet member magneticallypolarized to produce a magnetic field orthogonal to said longitudinalaxis.

18. The amplifier of claim 10 wherein said deflecting means is comprisedof at least one permanently magnetized bar magnet having its polesaligned along an imaginary line orthogonal to said longitudinal axis.

References Cited UNITED STATES PATENTS 3,394,282 7/1968 Schmidt 315-5352,955,225 10/19 60 Sterzer 3l5--3.5

HERMAN KARL SAALBACH, Primary Examiner. PAUL L. GENSLER, AssistantExaminer.

US. Cl. X.R.

